Lithium ion batteries, a member of the rechargeable secondary battery group, are able to provide high energy in comparison to their volume and weight. Lithium, a metal having a light and high oxidation potential, contributes to these batteries, thereby ensuring the batteries with a high gravimetric and volumetric energy density. Lithium ions are switched between the anode (negative electrode) and the cathode (positive electrode) and they move from anode to cathode during discharging and from cathode to anode during charging.
Lithium ion batteries have been initially widely preferred as a power source for consumer electronics (mobile phones, laptop computers, digital cameras, cameras, etc.) and other wireless, portable electronic devices. Moreover, lithium ion batteries are used in military applications and in the aviation field for operating of aircraft emergency systems. On the other hand, recent developments in lithium-ion battery technology have made it possible to use these batteries in electric vehicles in fixed energy storage systems and smart networks for the storage of renewable energy types such as solar energy and wind energy.
The polymer binders used in rechargeable batteries enable the active materials used in the electrodes to be connected to each other and allows the active material to be connected to the accumulator used. These binders are generally composed of one or more polymers. Commonly used binders in commercial lithium ion batteries are polyvinyldifluoride (PVDF) and ethylene-propylene diene. These polymers are soluble in organic solvents such as N-methyl pyrrolidone (NMP) because they are mostly water-insoluble. These organic solvents also allow the electrode active materials and other admixture materials if available, to be dispersed homogeneously in the solvent together. However, the organic solvents used are expensive and they can be harmful to the environment. Besides, as PVDF is not stable at high temperatures, its structure deteriorates.
Examples of water-soluble binders include carboxymethyl cellulose (CMC). In addition to CMC, polytetrafluoroethylene (PTFE) and styrene butadiene rubber are also water-soluble binders. However these binders reduce the cycle life of the electrodes when used because they provide low adhesion. During mass production, it is desirable to dissolve the binders in water, which is much cheaper than using organic solvent, and it is very important for the water-soluble binders to provide the desired performance to the electrode structure.
Nowadays the commercially available negative electrode is graphite as it presents structural stability during charging and discharging and low volumetric expansion during the reaction with lithium. Graphite forms the LiC6 compound with 1 lithium atom with 6 C atoms retained. This situation causes the theoretical capacity of the graphite material to be low (372 mAh/g). One of the approaches that have been tried in order to increase the capacity is to change the active materials used in electrodes. Due to their high theoretical capacities aluminium (Al), silicium (Si), tin (Sn), and antimone (Sb) are metals tried most as electrode active materials nowadays. However, it appears that the incorporation of lithium into the structure of these metals during conversion has led to a change in the lattice structures and a volume increase of more than 200% per unit cell. This situation leads to deterioration of the crystal structure and decrease of the capacity due to the increase of the lithium ratio in the lithium-active metal compound with progressive conversions. At the same time, this high amounts of volumetric change created leads to the increase of the internal tension of the electrode structure, resulting in fracture formation in the electrode structure.
Polymers used as binding agents in electrodes can maintain the structure, and suppress the internal tensions and volumetric expansions caused by the reaction of commercially available graphite with lithium. However, in metals that show more than 200% volumetric change following reaction with lithium that have been tried instead of graphite, the binding polymers (CMC, PVDF, EPDM) used for graphite do not have the binding capacity or flexibility that can tolerate volumetric changes presented due to the fact that metals arrest more lithium (Al, Si, Sn, Sb). For this reason, the use of polymers in such metals that exhibit these types of high volumetric changes as active materials in the electrodes of lithium ion batteries, is of great importance.
Another problem of the electrode active materials used in lithium ion batteries is that they are not sufficiently conductive and depending on this fact their electrode structure deteriorates in high charge and discharge rates. For this reason, additives that increase the conductivity are added to electrode active materials. These additives can be carbon-based materials (such as carbon black, graphene, super-P conductor) as well as conductive polymers (PT, PANI). However, in order to increase conductivity in addition to the polymers used as binders, the addition of carbon-based materials increases both cost and battery weight. Therefore, nowadays it important to provide conductivity to polymers used as binders.
It is important therefore for the polymer or polymers that are used, to be conductive and moreover water soluble in terms of production costs.
In order for the binding polymers to maintain the structure in spite of the defined volume changes, their fluorescence characteristics have to be developed. In order for a high conductive characteristics, groups such as pyrrole and thiophene need to be present inside the polymer.
Patent documents such as U.S. Pat. No. 8,852,461 B2, WO2014026112 A1, U.S. Pat. Nos. 7,311,997 B2 and 6,300,015 can be given as examples to the known state of the art. However, these documents are different to the invention in many aspects. Moreover, the methods applied in the present technique are not satisfactory in terms of being cost-effective.